Discovery of a heavy silicon isotope mantle reservoir

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Abstract
Silicon cycling between Earth’s reservoirs provides critical insights into how the Earth operates. While average crust and the bulk silicate Earth (BSE) share similar silicon isotope (δ30Si) compositions, some mantle-derived magmas exhibit lower δ30Si values than the BSE, implying the existence of an unidentified mantle reservoir with complementary higher δ30Si values. We present silicon isotope data from Cenozoic lamproites and their hosted mantle pyroxenite xenoliths from the Himalaya–Tibet orogen. These mantle-derived rocks have higher δ30Si values than the BSE, which resulted from reaction between mantle peridotite and 30Si-rich silicate melts from subducted Indian continental crust. Our results demonstrate that slab melting can produce high-δ30Si melts and complementary low-δ30Si residues. These products are unevenly distributed in the mantle, with high-δ30Si melts stored as metasomatic veins in the lithospheric mantle while low-δ30Si residues are recycled into the deep mantle. This study provides evidence that mantle metasomatism by high-δ30Si slab melts creates heavy silicon reservoirs in the lithospheric mantle above continental slabs or cool mantle wedges above oceanic slabs.
Funding Information
  • National Science and Technology Major Project (2024ZD1001103)
  • National Natural Science Foundation of China (42021002, 42472073)

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